achievements

I just finished the Portal 2 single player mode, but there are some achievements in some levels of the story mode that I didn't get. Is there any way to go back to a specific level without replaying the whole game?

You can't go back to a specific puzzle, but you can choose to start a specific chapter from the main menu. You will have to play the chapter until you get to the specific puzzle. What achievements were they? Maybe I can help.